1. Field of the Invention
The invention relates to a rubber gasket type joint for pressure pipe and more particularly one for use with large diameter flexible pipe.
2. Description of the Prior Art
The pipe coupling art is well developed. For example, in the rigid pipe art, i.e., concrete pipe, bell and spigot interconnections wherein a rubber gasket is utilized as the sealing means are shown in U.S. Pat. No. 2,223,434 to Trickey; U.S. Pat. No. 2,576,012 to Gurck; and U.S. Pat. No. 3,129,961 to Danko, et al. and are exemplary of the art. However, it is important to note that sustained watertightness of each joint configuration above disclosed is highly dependent upon the rigidity of the barrel of the pipe to resist deflection so as to maintain required gasket compression and prevent formation of excessive clearance (between the exterior periphery of the heel shoulder forming the gasket recess and the interior surface of the bell) through which the gasket might be extruded by internal pressure and thus become displaced. While such joints have been utilized successfully for semi-rigid pipe of moderate diameter and for semi-flexible pipe of lesser diameter, they have proved to be unsatisfactory for large diameter flexible pipe wherein required joint tolerances are most difficult to maintain; bell and/or spigot ends are frequently out of round, or ever worse, contain flattened portions and finally; (following subgrade preparation, laying of the pipe, joint assembly, embedment and backfill), the spigot or the bell end of the joint may be subjected to the effects of (1) uneven vertical overfill load, (2) differential subgrade settlement, and (3) transverse shear through the joint resulting from (1) and/or (2) above which is most likely to cause radial movement with respect to one another.
An ideal rubber gasketed joint would include metal to metal contact between spigot shoulder and bell surface around 360.degree. of circumference. In this instance an external force acting inwardly on the spigot end of one pipe, such as a rock protruding from the subgrade, would be immediately resisted and radial clearance which might otherwise be caused between spigot shoulder and bell surface would be minimal. In order to deflect inwardly, a portion of the circumference of the truly circular spigot must become flattened. But since the circumference of the spigot would then necessarily be shorter, the force causing such flattening must be sufficient to compress the spigot metal circumferentially: a very large force, indeed!While the above described ideal joint embodies several desirable features, assurance of metal to metal fit for interchangeable parts would prohibit use of required manufacturing tolerances. Additionally, lacking clearance between the parts, field assembly would be impractical if not impossible. However, if and when clearance between the parts is provided to accommodate manufacture and to facilitate assembly the above described force required to cause flattening is greatly reduced. The only circumferential resistance offered would be sliding friction between the parts. This invention permits use of generous manufacturing tolerances and initial clearances, and yet after grouting provides the beneficial effects resulting from metal to metal contact between the parts.
Thus, for large pipes flexible pipes complex and expensive couplings are required to accommodate this relative movement between pipe sections. A successful example of this is the Dresser coupling which has many parts mounted externally of the pipe interconnected by a plurality of bolts based around the coupling for drawing it tight. While rubber gasket joints have been used, whenever the pipe is out of round such that the bell and spigot do not match up to assure that the gasket will touch at all locations, there will be leakage and in some instances if the pressures are sufficient the gasket will be extruded through the space between the bell and spigot causing further leakage problems. There are many conditions which can cause pipe ends to be out of round during and subsequent to the time the joint is assembled. For example, the out of round condition might occur (a) during manufacture, handling, loading, transporting, unloading, placing and assembly in a trench, (b) due to a non-uniform bed under adjacent pipe sections, and (c) due to non-uniform covering over the pipe which creates a non-uniform load over adjacent pipe sections, and may be expected whenever pipe passes through a retaining wall or starts into the side of a hill. Also, after assembly, distortions can occur due to uneven settling of the bedding or other conditions which create shifting of the pipe sections or uneven loading on them. If the joint is not held securely under such conditions, deflections at the joint may occur which result in leakage.
Welded joints for flexible pipe are also well known and are satisfactory in some environments but they can rupture due to expansion of the pipe or due to movement of the earth or due to earthquakes and the like. For this reason, welded joints cannot be used in areas where earth movement is likely. Furthermore, welding is quite expensive and therefore often economically unfeasible.